Echo cancellation devices are well known. When a loudspeaker and a microphone are positioned close together and used simultaneously, as in (hands-free) telephones, part of the far-end signal appears as an echo in the microphone signal. A typical echo cancellation device comprises an adaptive filter that models the acoustic path between the loudspeaker rendering the far-end signal and the microphone receiving both the echo and the near-end signal. If the echo cancellation signal produced by the adaptive filter is equal to the echo in the microphone signal, the echo can be cancelled out and only the near-end signal remains. However, the residual signal resulting from combining the microphone signal and the echo cancellation signal typically still contains echo components. To remove such echo components, a post-processor may be used to further process the residual signal and remove any remaining echo components. The post-processor unit typically provides a time and frequency dependent gain function that selectively attenuates those frequencies at which a significant residual far-end echo is present.
U.S. Pat. No. 6,546,099 (Philips) discloses an acoustic echo cancellation device which includes a post-processor. This Prior Art echo cancellation device further includes a spectrum estimator for determining the frequency spectrum of the echo cancellation signal. The post-processor comprises a filter which is dependent on the frequency spectrum of the echo cancellation signal. The use of such a post-processor significantly improves the suppression of the remaining echo in the residual signal.
The arrangement known from U.S. Pat. No. 6,546,099 performs well in most cases. However, in some circumstances the remaining echo cannot be sufficiently suppressed without suppressing or distorting the near-end signal. This may be the case in a so-called double-talk situation, where far-end speech and near-end speech are simultaneously received by the microphone, especially when the far-end speech is relatively loud. In modern mobile (cellular) telephone apparatus this is often the case as the loudspeaker and the microphone are located very close together. When used in hands-free mode, the far-end echo may be much louder than the near-end signal, causing Prior Art echo cancellation devices to introduce audible signal distortions.
This problem is aggravated when the echo is not only caused by the far-end signal but also by an additional signal, such as a music signal or a second far-end signal. Modern mobile telephone apparatus, for example, are often capable of reproducing music downloaded from the Internet, for example music stored in MP3 format or a similar format. When this music is reproduced during a telephone call, a situation of continuous double-talk results. Prior Art acoustic echo cancellation devices fail to offer a solution to this problem.